Time-of-flight imaging method, device and magnetic resonance imaging system

Abstract

The embodiments of the present invention disclose a time-of-flight imaging method, device, and magnetic resonance system. The method includes: in EPI imaging with multiple excitations, performing flow compensation in the direction of layer selection, phase, and frequency encoding for the echo of each excitation; performing two scans for each excitation, and using readout gradients with the same or opposite polarity to acquire the flow-compensated echoes in the two scans, and each acquisition starts from the mth echo and ends at the last echo, m = N-"α*N"+1, N is the total number of echoes acquired in each excitation when a linear reordering method is used, "" is a round-up operator, α is the partial Fourier parameter value of the set phase encoding direction, and α<1; for each excitation, averaging the echoes at the same position acquired in the two scans to obtain the echo data corresponding to this excitation; and performing TOF imaging on the acquired echo data. The embodiments of the present invention reduce the echo acquisition time.

Description

Technical Field

[0001] The present invention relates to the field of MRI (Magnetic Resonance Imaging) technology, in particular to a TOF (Time Of Flight) method, device and MRI system. Background Art

[0002] MRI applies radio frequency (RF) pulses of a specific frequency to the human body in a static magnetic field, exciting the hydrogen protons in the human body and causing MR (Magnetic Resonance) phenomenon. After the pulse is stopped, the protons generate MR signals during the relaxation process. MR images are generated through the processing of MR signal reception, spatial encoding and image reconstruction.

[0003] During the imaging process, after RF excitation, slice selection is first performed using the Z-direction gradient, effectively selecting a slice for imaging. The Y-direction gradient magnetic field is then applied, causing the precession velocities of the magnetic moment at different Y positions to differ. Disabling the Y-direction gradient restores the magnetic moment velocities to the same value at all positions, but the phase offsets at different Y positions differ due to the previously varying precession velocities. This process is called phase encoding. Next, the X-direction gradient is applied, causing the velocities of the magnetic moment at different X positions to differ. This process is called frequency encoding. Through phase and frequency encoding, each position in a 2D image can be determined. The signals collected by the receiving coils are k-space signals, which are then converted into an image through Fourier transform. A new round of slice selection is then performed using the Z-direction gradient, and the above process is repeated to produce a 3D MRI image.

[0004] Different RFs can be formed by combining 90°RF and 180°RF. Commonly used MRI RFs include: SE (Spin-Echosequence), TSE (Turbo Spin-Echo sequence), GRE (Gradient-Echo sequence) and EPI (Echo Planar Imaging). Among them, GRE and EPI are both gradient echoes. The generation of gradient echo is that after one RF excitation, the direction of the readout gradient field (i.e., the X direction) is switched forward and backward to generate a gradient echo. The difference between EPI and general gradient echo is that after one RF excitation, the readout gradient field is continuously switched forward and backward, and each switch generates a gradient echo, so there is an echo chain, which is similar to TSE.

[0005] In magnetic resonance imaging, the phase accumulation of stationary tissue after experiencing slice selection gradients of equal magnitude and opposite direction (i.e., Z-direction gradient) is zero. However, the phase accumulation of moving tissue, such as flowing blood and cerebrospinal fluid, moving along the direction of the readout gradient field (i.e., X-direction) is not zero. When the subsequent phase encoding gradient (i.e., Y-direction gradient) is applied, the moving tissue that already has a phase will be incorrectly encoded and appear in other locations, becoming a flow artifact.

[0006] FC (Flow Compensation), also known as GMN (Gradient Moment Nulling), is a technology that uses specially designed gradient fields to reduce or eliminate flow artifacts. FC technology has many gradient combination modes. By repeatedly transforming positive and negative gradient fields of different areas, the phase offset of fluids at various speeds can eventually approach zero, thereby eliminating motion artifacts. Generally, FC compensates for the effects of uniform flow by eliminating the first-order gradient moment. The GMN mentioned in this article also refers to first-order moment compensation.

[0007] TOF imaging is based on gradient echo scanning with flow compensation. This imaging technique relies on the flow enhancement effect to distinguish between moving and stationary protons to form MRI images.

[0008] TOF imaging is a non-contrast magnetic resonance angiography (MR Angiography, MRA) method widely used in clinical practice, and usually adopts 2D or 3D-GRE sequences. Since the 3D-GRE sequence can better depict the arterial tree, it is most commonly used for intracranial angiography. The fast selective RF excitation pulse in the 3D-GRE sequence can suppress the background signal from static tissue, while the saturation band on the venous side of the imaging area can suppress the venous signal. Traditional 3D-TOF imaging uses the tracking venous saturation band at the end of the imaging plate to remove venous contamination in TOF imaging, and has a high SAR (Specific Absorption Ratio), especially in high-field scanning. In addition, due to the low scanning efficiency, TOF imaging based on 3D-GRE usually requires a long scanning time. Summary of the Invention

[0009] In view of this, the embodiments of the present invention provide a TOF imaging method on the one hand, and a TOF imaging device and an MRI system on the other hand, so as to reduce the echo acquisition time without affecting the quality of magnetic resonance imaging.

[0010] A time-of-flight (TOF) imaging method, comprising:

[0011] In multiple-shot echo planar EPI imaging, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for each shot echo.

[0012] Two scans are performed for each excitation, and the two scans use readout gradients of the same or opposite polarity to acquire flow-compensated echoes. Each acquisition starts from the mth echo and ends at the last echo, where m = N - "α*N" + 1, where N is the total number of echoes acquired per excitation using the linear reordering method, "" is the ceiling operator, and α is the partial Fourier parameter value set in the phase encoding direction, and α<1;

[0013] For each excitation, the echoes at the same position acquired during the two scans are averaged to obtain the echo data corresponding to this excitation;

[0014] The TOF imaging is performed on the echo data corresponding to each excitation.

[0015] α≥4 / 8.

[0016] The values of N and α satisfy: N is an odd number, and "α*N" is an odd number.

[0017] The flow compensation in the slice encoding direction, phase encoding direction and frequency encoding direction for each excited echo includes:

[0018] Performing flow compensation in the slice encoding direction and the phase encoding direction for the central echo of each excitation, wherein the central echo is the central echo of multiple echoes to be collected after each excitation when a linear reordering method is used;

[0019] Flow compensation in the frequency encoding direction is performed on the odd-numbered echoes of each excitation.

[0020] The flow compensation in the slice selection encoding direction and the phase encoding direction for the central echo of each excitation includes:

[0021] Flow compensation in the layer selection coding direction and phase coding direction is performed according to the following formula:

[0022] M 1,par =M 0,par Δt par

[0023]

[0024] Among them, M 1,par is the first-order moment of the layer selection coding direction at the center echo, M 1,phase is the first-order moment of the phase encoding direction at the center echo; M 0,paris the zero-order moment of the layer-selected coding gradient in the layer-selected coding direction, M 0,phaseprephase is the zero-order moment of the pre-phase gradient in the phase encoding direction, M 0,par 、M 0,phaseprephase is different in different excitations; Δt par Δt is the time from the center of the layer coding gradient to the center of the echo in the layer coding direction, p Δt is the time from the center of the pre-phase gradient to the center of the echo in the phase encoding direction, pk M is the time from the encoding gradient of the kth echo in the phase encoding direction to the central echo; 0,pk is the zero-order moment of the phase encoding gradient of the k-th echo of a shot, k center It is the sequence number of the center echo to be collected when the linear reordering method is used.

[0025] A time-of-flight (TOF) imaging method, comprising:

[0026] In multiple-shot echo planar EPI imaging, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for each shot echo.

[0027] Determining a plurality of echoes to be acquired after each excitation when a linear reordering method is used, and for each excitation, acquiring each echo starting from a central echo of the plurality of echoes in a positive or negative direction of k-space, wherein the echo acquisition direction for the current excitation is opposite to the echo acquisition direction for the previous excitation;

[0028] Perform TOF imaging on the collected echo data.

[0029] The echo collection direction for the current excitation is opposite to the echo collection direction for the previous excitation, including:

[0030] When the echo acquisition direction for the previous excitation is: starting from the central echo to acquire in the positive direction of k-space to the last echo when acquired in a linear reordering manner, the echo acquisition direction for the current excitation is: starting from the central echo to acquire in the negative direction of k-space to the first echo when acquired in a linear reordering manner; or,

[0031] When the echo acquisition direction for the previous excitation is: starting from the central echo to the negative direction of k-space to the first echo when acquired in a linear reordering manner, the echo acquisition direction for this excitation is: starting from the central echo to the positive direction of k-space to the last echo when acquired in a linear reordering manner.

[0032] The collecting each echo in a positive or negative direction of the k-space starting from the central echo of the plurality of echoes includes:

[0033] When the selective slice encoding gradient pulse and the phase encoding gradient pulse for the current excitation are transmitted completely, starting from the central echo of the multiple echoes, each echo is acquired in the positive or negative direction of the k-space.

[0034] After acquiring each echo starting from the central echo of the multiple echoes in the positive or negative direction of the k-space and before performing TOF imaging on the acquired echo data, it further includes:

[0035] Averaging the superposition of the two central echoes acquired in the m-th and (m + 1)-th acquisitions, where 1 ≤ m < M, and M is the total number of excitations for the current EPI imaging.

[0036] The flow compensation for the selective slice encoding direction, phase encoding direction, and frequency encoding direction for the echo of each excitation includes:

[0037] Performing flow compensation for the selective slice encoding direction and phase encoding direction on the central echo of each excitation, where the central echo is the central echo of the multiple echoes to be acquired when using the linear reordering method after each excitation;

[0038] Performing flow compensation for the frequency encoding direction on the odd-numbered echo of each excitation.

[0039] The flow compensation for the selective slice encoding direction and phase encoding direction on the central echo of each excitation includes:

[0040] M 1,par = M 0,par Δt par

[0041] M 1,phase = M 0,phaseprephase Δt p

[0042] where M 1,par is the first moment of the selective slice encoding direction at the central echo, M 1,phase is the first moment of the phase encoding direction at the central echo; M 0,par is the zero moment of the selective slice encoding gradient in the selective slice encoding direction, M 0,phaseprephase is the zero moment of the pre-phase gradient in the phase encoding direction, M 0,par 、M 0,phaseprephase are different in different excitations; Δt par is the time from the center of the selective slice encoding gradient to the center of the echo in the selective slice encoding direction, Δt p is the time from the center of the pre-phase gradient to the center of the echo in the phase encoding direction.

[0043] A time-of-flight (TOF) imaging device, comprising:

[0044] Flow compensation module: In multi-shot echo planar EPI imaging, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for each echo shot;

[0045] Echo acquisition module: Two scans are performed for each excitation. The two scans use readout gradients with the same or opposite polarity to acquire flow-compensated echoes. Each acquisition starts from the mth echo of the excitation and ends at the last echo. m = N - "α*N" + 1, where N is the total number of echoes per excitation, "" is the round-up operator, and α is the partial Fourier parameter value set in the phase encoding direction, α < 1.

[0046] Echo averaging module: For each excitation, the echoes at the same position acquired during two scans are averaged to obtain the echo data corresponding to this excitation;

[0047] Imaging module: performs TOF imaging on the echo data corresponding to each excitation.

[0048] A time-of-flight (TOF) imaging device, comprising:

[0049] Flow compensation module: In multi-shot echo planar EPI imaging, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for each echo shot;

[0050] Echo acquisition module: determines the multiple echoes to be acquired after each excitation when using the linear reordering method. For each excitation, each echo is acquired starting from the center echo of the multiple echoes in the positive or negative direction of k-space. The echo acquisition direction for this excitation is opposite to the echo acquisition direction for the previous excitation.

[0051] Imaging module: performs TOF imaging on the collected echo data.

[0052] A magnetic resonance imaging system comprises any one of the above-mentioned time-of-flight TOF imaging devices.

[0053] In an embodiment of the present invention, when two scans are performed for one excitation, only a portion of the echoes are collected in each scan, and the starting echo to be collected is as close as possible to the center of the echo to be collected when the linear reordering method is used. Therefore, while ensuring the quality of magnetic resonance imaging, the echo acquisition time is shortened as much as possible, and the flow artifact and SAR are reduced. Alternatively, the multiple echoes collected in one excitation when the linear reordering method is used are split into two adjacent excitations when the center reordering method is used for acquisition. Since only a portion of the echoes are collected in each excitation, the echo acquisition time is greatly shortened, the flow artifact and SAR are reduced, and the two adjacent excitations are used to obtain the same complete echo as the one obtained by the linear reordering method. Therefore, the image resolution is not reduced, and the image quality is guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, so that those skilled in the art will understand the above and other features and advantages of the present invention more clearly. In the accompanying drawings:

[0055] Figure 1 A flow chart of a TOF imaging method provided by one embodiment of the present invention;

[0056] Figure 2 : A schematic diagram of a 3D segmented EPI sequence used in the first embodiment of the present invention is shown in FIG.

[0057] Figure 3 A flow chart of a TOF imaging method provided in accordance with a second embodiment of the present invention;

[0058] Figure 4 3D segmented EPI sequence used in two adjacent excitation processes (mth and m+1th) in the second embodiment of the present invention;

[0059] Figure 5 To adopt Figure 4 The trajectory diagram of the echo acquisition process in K space after the 3D segmented EPI sequence is excited;

[0060] Figure 6 Schematic diagram comparing MIP images obtained by respectively using an existing 3D-GRE-based TOF imaging scheme, a TOF imaging scheme based on a 3D segmented EPI using partial Fourier transforms and not using alternating readout provided by the first embodiment of the present invention, and a TOF imaging scheme based on a 3D segmented EPI using partial Fourier transforms and using alternating readout provided by the first embodiment of the present invention;

[0061] Figure 7Schematic diagram comparing MIP images obtained by using an existing 3D-GRE-based TOF imaging solution and a TOF imaging solution based on center-reordered 3D segmented EPI provided by the second embodiment of the present invention;

[0062] Figure 8 A schematic structural diagram of a TOF imaging device provided by a first embodiment of the present invention;

[0063] Figure 9 This is a schematic structural diagram of a TOF imaging device provided in the second embodiment of the present invention.

[0064] The accompanying drawings are numerals as follows:

[0065] Label meaning 101～104 step 301～303 step 81 Flow compensation module according to the first embodiment of the present invention 82 Echo acquisition module of the first embodiment of the present invention 83 Echo averaging module according to the first embodiment of the present invention 84 Imaging module according to the first embodiment of the present invention 91 Flow compensation module according to the second embodiment of the present invention 92 Echo acquisition module according to the second embodiment of the present invention 93 Imaging module according to the second embodiment of the present invention DETAILED DESCRIPTION

[0066] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail with reference to the following examples.

[0067] Figure 1 A flow chart of a TOF imaging method according to an embodiment of the present invention is provided, wherein the specific steps are as follows:

[0068] Step 101: In multi-shot EPI imaging, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for the echo of each shot.

[0069] In an optional embodiment, flow compensation in the layer selection coding direction, phase coding direction and frequency coding direction is performed for the echo of each excitation, including: flow compensation in the layer selection coding direction and phase coding direction is performed for the central echo of each excitation, wherein the central echo is the central echo of multiple echoes to be collected when a linear sorting method is adopted after each excitation; and flow compensation in the frequency coding direction is performed for the odd-numbered echo of each excitation.

[0070] Step 102: Two scans are performed for each excitation. The two scans use readout gradients with the same or opposite polarity to acquire flow-compensated echoes. Each acquisition starts from the mth echo of the excitation and ends with the last echo, m = N - "α*N" + 1, where N is the total number of echoes acquired for each excitation using a linear reordering method, "" is a round-up operator, and α is the partial Fourier parameter value of the set phase encoding direction, α<1.

[0071] In an optional embodiment, 4 / 8≤α<1, for example, α can be 4 / 8 or 5 / 8 or 6 / 8 or 7 / 8.

[0072] In an embodiment of the present invention, the center echo to be collected after each excitation can be limited to an odd-numbered echo, so that the center echo can always obtain complete flow compensation in three directions. Specifically, by selecting the values of N and α, N and "α*N" can be odd numbers, so that the center echo to be collected after each excitation is an odd-numbered echo. For example: when the number of echoes to be collected after each excitation is set to N = 9 when a linear reordering method is used, and the partial Fourier parameter value α in the phase encoding direction is set to 6 / 8, then "α*N" = "9*6 / 8" = 7, and the center echo to be collected after each excitation is the third echo.

[0073] Step 103: For each excitation, the echoes at the same position acquired by two scans are averaged to obtain the echo data corresponding to this excitation.

[0074] Step 104: Perform TOF imaging on the obtained echo data corresponding to each excitation.

[0075] The beneficial technical effects of the above embodiment are as follows:

[0076] First, when performing two scans for a single excitation, only a portion of the echo is acquired in each scan, and the starting echo is as close as possible to the echo center of the echo to be acquired using the linear reordering method. Therefore, while maintaining the quality of magnetic resonance imaging, the echo acquisition time is minimized, and the flow artifact and SAR are reduced.

[0077] Second, because flow compensation is performed in the slice encoding, phase encoding, and frequency encoding directions for each echo in multiple-shot EPI imaging, and an efficient acquisition scheme is employed, scanning efficiency can be improved while ensuring flow compensation. Furthermore, due to the short echo interval, high scanning efficiency and minimal distortion can be maintained, allowing for more flexible selection of echo lengths.

[0078] Third, for one excitation, two scans are performed using readout gradients with opposite polarities to further reduce the phase oscillation between odd and even echoes after each excitation.

[0079] Figure 2 FIG shows a schematic diagram of a 3D segmented EPI sequence used in the first embodiment of the present invention.

[0080] For each excitation, flow compensation gradients Sg, Pg, and Fg are applied in the directions of slice encoding (PA), phase encoding (PE), and frequency encoding (FE), and the calculation of the flow compensation gradients Sg and Pg is based on the central echo Ec among the multiple echoes of each excitation.

[0081] Among them, the flow compensation in the slice selection gradient direction does not involve multiple echoes, so it can be calculated directly based on the slice selection gradient, just like 3D-GRE.

[0082] In an optional embodiment, for the central echo among the multiple echoes of each excitation, flow compensation in the slice selection encoding direction and the phase encoding direction is performed according to the following formula:

[0083] M 1,par =M 0,par Δt par

[0084]

[0085] Among them, M 1,par is the first-order moment of the layer selection coding direction at the center echo, M 1,phase is the first-order moment of the phase encoding direction at the center echo; M 0,par is the zero-order moment of the layer-selected coding gradient in the layer-selected coding direction, M 0,phaseprephase is the zero-order moment of the pre-phase gradient in the phase encoding direction, M 0,par 、M 0,phaseprephase is different in different excitations; Δt par Δt is the time from the center of the layer coding gradient to the center of the echo in the layer coding direction, p Δt is the time from the center of the pre-phase gradient to the center of the echo in the phase encoding direction, pk M is the time from the encoding gradient of the kth echo in the phase encoding direction to the central echo; 0,pk is the zero-order moment of the phase encoding gradient of the k-th echo of a shot, k center It is the sequence number of the center echo to be collected when the linear reordering method is used.

[0086] Figure 2 In the embodiment, the total number of echoes collected in each excitation when the linear reordering method is used is limited to N=9, and the partial Fourier parameter α in the phase encoding direction is set to 6 / 8. Then the number of echoes collected in each scan (i.e., EPI factor) = "9*6 / 8" = 7, m = 9-7+1 = 3, and each scan starts collecting from the 3rd echo to the last echo, i.e., the 9th echo, for a total of 7 echoes.

[0087] Depend on Figure 2It can be seen that the readout gradients used in the two scans have opposite polarities. Specifically, the readout gradient applied in the first scan A1 is: negative, positive, negative, positive, negative, positive, negative, and the readout gradient applied in the second scan A2 is: positive, negative, positive, negative, positive, negative, positive. The echoes at the same position collected in the two scans are averaged to obtain the echo data corresponding to this excitation. The echoes at the same position are, for example, the 3rd echo collected in the first scan and the 3rd echo collected in the second scan, the 4th echo collected in the first scan and the 4th echo collected in the second scan, and so on.

[0088] Figure 3 The following is a flowchart of the TOF imaging method provided by the second embodiment of the present invention, and the specific steps are as follows:

[0089] Step 301: In the echo planar EPI imaging with multiple excitations, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for the echoes of each excitation.

[0090] Step 302: Determine the multiple flow-compensated echoes to be collected when using the linear reordering method after each excitation. For each excitation, starting from the central echo of the determined multiple echoes, each echo is collected in the positive or negative direction of the k-space. The echo collection direction for this excitation is opposite to that for the previous excitation.

[0091] Among them, the echo collection direction for this excitation being opposite to that for the previous excitation can be one of the following two:

[0092] One, when the echo collection direction for the previous excitation is: starting from the central echo and collecting to the last echo when collected in the positive direction of the k-space in the linear reordering method, the echo collection direction for this excitation is: starting from the central echo and collecting to the first echo when collected in the negative direction of the k-space in the linear reordering method;

[0093] Two, when the echo collection direction for the previous excitation is: starting from the central echo and collecting to the first echo when collected in the negative direction of the k-space in the linear reordering method, the echo collection direction for this excitation is: starting from the central echo and collecting to the last echo when collected in the positive direction of the k-space in the linear reordering method. Step 303: Perform TOF imaging on the collected echo data.

[0094] Since the central echo is collected in the echo collection process of two adjacent excitations, before performing TOF imaging, the two central echoes collected in the mth and (m + 1)th acquisitions can be superimposed and then averaged, where 1 ≤ m < M, and M is the total number of excitations in this EPI imaging.

[0095] For the convenience of distinction and description, the solution provided in the first embodiment of the present invention is referred to as a TOF imaging solution based on partial Fourier 3D segmented EPI, and the solution provided in the second embodiment of the present invention is referred to as a TOF imaging solution based on center reordering 3D segmented EPI.

[0096] The beneficial technical effects of the above embodiment are as follows:

[0097] First, by splitting the multiple echoes acquired in a single shot using linear reordering into two adjacent shots using central reordering, echo acquisition time is greatly shortened, flow artifacts, and SAR are reduced because each shot only acquires a portion of the echoes. Furthermore, due to the short echo interval, high scanning efficiency and minimal distortion can be maintained. Furthermore, the two adjacent shots produce the same complete echoes as a single shot using linear reordering, thus maintaining image resolution and ensuring image quality.

[0098] Second, because the acquisition starts from the center echo after each excitation, the influence of flow effect on TOF image is reduced, and because the center echo is acquired once in two adjacent excitation processes, the signal-to-noise ratio of TOF image is improved;

[0099] Third, partial echoes are obtained by sampling the k-space in the positive and negative directions in two consecutive excitations with opposite phase encoding directions, which provides the feasibility of distortion correction using the standard reverse gradient method, which will allow a more flexible choice of echo sequence length.

[0100] In an embodiment of the present invention, the total number of echoes collected in each excitation when a linear reordering method is used can be limited to an odd number, so that the center echo is the middle echo. For example: if it is determined that the number of echoes to be collected in each excitation when a linear reordering method is used is 9, then the center echo is the 5th echo.

[0101] In an optional embodiment, flow compensation in the layer selection coding and phase coding directions is performed on the central echo of each excitation. The central echo is the central echo of multiple echoes to be collected when a linear reordering method is adopted after each excitation; flow compensation in the frequency coding direction is performed on the odd-numbered echoes (i.e., the 1st, 3rd, 5th, 7th, ...) of each excitation.

[0102] The flow compensation in the direction of slice selection encoding and phase encoding can be performed on the central echo of each excitation according to the following formula:

[0103] M 1,par =M 0,par Δt par

[0104] M 1,phase =M0,phaseprephase Δt p

[0105] Among them, M 1,par is the first-order moment of the layer selection coding direction at the center echo, M 1,phase is the first-order moment of the phase encoding direction at the center echo; M 0,par is the zero-order moment of the layer-selected coding gradient in the layer-selected coding direction, M 0,phaseprephase is the zero-order moment of the pre-phase gradient in the phase encoding direction, M 0,par 、M 0,phaseprephase is different in different excitations; Δt par is the time from the center of the layer coding gradient to the center echo in the layer coding direction, Δt p is the time from the center of the pre-phase gradient to the center echo in the phase encoding direction. In this embodiment, the first echo in the echo chain acquired using the center reordering is the center echo when acquired using the linear reordering method.

[0106] Echo acquisition using linear reordering is a mature technology. In this embodiment, the center echo refers to the center echo of multiple echoes acquired using linear reordering after each excitation. For example, if nine echoes are acquired using linear reordering after each excitation, the fifth echo is the center echo.

[0107] To further reduce the impact of flow effects on TOF images, in a preferred embodiment, after the slice-selective encoding gradient pulses and phase-encoding gradient pulses are transmitted, each echo is acquired from the central echo in the positive or negative direction of k-space. In other words, the echo acquisition process begins immediately after the slice-selective encoding gradient pulses and phase-encoding gradient pulses are transmitted.

[0108] In this embodiment, the total number of echoes collected during each excitation using linear reordering can be limited to an odd number, such that the center echo is the middle echo. For example, if the number of echoes to be collected during each excitation using linear reordering is determined to be 9, then the center echo is the fifth echo. Furthermore, it should be noted that in this embodiment, when using the center reordering method to collect echoes, symmetry must be maintained between adjacent excitations. That is, the same number of echoes must be collected symmetrically in the positive and negative directions of k-space, starting from the center echo, during the two adjacent excitations.

[0109] Figure 4This is an example diagram of the 3D segmented EPI sequence used in two adjacent excitation processes (the mth and m+1th) of the second embodiment of the present invention. It can be seen that for each excitation, flow compensation gradients Sg, Pg, and Fg are applied in the slice encoding (PA), phase encoding (PE), and frequency encoding (FE) directions. The calculation of the flow compensation gradients Sg and Pg is based on the center echo Ec among the multiple echoes of each excitation. In this example, when a linear reordering method is used for acquisition in one excitation, the number of echoes collected is 9. The center echo EC, that is, the 5th echo, is the first echo collected after each excitation in this example.

[0110] Figure 5 To adopt Figure 4 The trajectory diagram of the echo acquisition process in K space after the 3D segmented EPI sequence is excited. Figure 5 As shown in the figure, if linear reordering is used after one excitation to collect 9 echoes, then in this example, after adopting the center reordering method, it will be split into 2 excitations, that is, in the mth excitation, starting from the center echo a5, a6, a7, a8 and a9 are collected in sequence. In the m+1th excitation, starting from the center echo b5, b4, b3, b2 and b1 are collected in sequence.

[0111] To evaluate the effectiveness of the 3D segmented EPI TOF imaging scheme based on partial Fourier transforms (PFT) provided in the first embodiment of the present invention and the 3D segmented EPI TOF imaging scheme based on center reordering provided in the second embodiment of the present invention, a 1.5T scanner equipped with a 20-channel head and neck coil was used in one example to test a healthy volunteer using a conventional 3D-GRE TOF imaging scheme, the PFT-based 3D segmented EPI TOF imaging scheme provided in the first embodiment of the present invention, and the center reordering TOF imaging scheme provided in the second embodiment of the present invention. Specific imaging parameters are shown in Table 1. The TR (time repetition) and flip angle in the 3D segmented EPI sequences were adjusted to match the background contrast in the 3D-GRE TOF imaging. Both 3D segmented EPI sequences used MOTSA (Multiple Overlapping Thin Slab Acquisition). After echo data acquisition, all MRA images were displayed using the maximum intensity projection (MIP) algorithm.

[0112]

[0113]

[0114]

[0115] Table 1

[0116] It can be seen that compared with the existing TOF imaging scheme based on 3D-GRE, the TOF imaging scheme based on partial Fourier 3D segmented EPI provided by the first embodiment of the present invention and the TOF imaging scheme based on center reordering 3D segmented EPI provided by the second embodiment of the present invention can significantly shorten the echo acquisition time.

[0117] Figure 6 A schematic diagram comparing MIP images obtained using an existing 3D-GRE-based TOF imaging scheme, a TOF imaging scheme based on 3D segmented EPI using partial Fourier transforms and not using alternating readout (i.e., two scans per excitation use readout gradients of the same polarity) provided by the first embodiment of the present invention, and a TOF imaging scheme based on 3D segmented EPI using alternating readout (i.e., two scans per excitation use readout gradients of opposite polarity) provided by the first embodiment of the present invention are provided. Among them:

[0118] a contains the upper, middle, and lower images in the leftmost column, corresponding to the existing 3D-GRE-based TOF imaging solution with an echo acquisition time of TA = 4 min 25 s (4 minutes and 25 seconds);

[0119] b includes the upper, middle, and lower images in the middle column, corresponding to the TOF imaging scheme provided by the first embodiment of the present invention using 3D segmented EPI based on partial Fourier transforms without alternating readout (i.e., two scans per excitation use readout gradients of the same polarity), with an echo acquisition time of TA = 2 minutes and 25 seconds.

[0120] Figure c includes the upper, middle, and lower figures in the rightmost column, corresponding to the TOF imaging scheme provided by the first embodiment of the present invention, which is based on 3D segmented EPI based on partial Fourier and adopts alternating readout (i.e., two scans for each excitation use readout gradients with opposite polarity), and the echo acquisition time TA = 2 min 25 s (2 minutes 25 seconds).

[0121] From images a, b, and c, we can see that:

[0122] Images a, b, and c show similar intracranial vascular appearances, with no significant flow artifacts observed in images b and c. Therefore, the TOF imaging solution based on 3D segmented EPI using partial Fourier transforms, as provided in Example 1 of the present invention, not only reduces echo acquisition time but also mitigates flow artifacts and increases vascular intensity. Although the use of the 3D segmented EPI sequence results in some signal-to-noise ratio loss, this has a minimal impact on vascular signals, resulting in images b and c being similar to image a.

[0123] In addition, the white arrow in the top image of b indicates a slight flow artifact in b, while the white arrow in the top image of c indicates a reduced artifact in c. Furthermore, the white arrows in the middle row of images and the bottom image of b and c indicate the same area, demonstrating improved background suppression in c. In other words, the alternating readout scheme in c improves both flow artifacts and background suppression, likely due to the reduced phase oscillation between odd and even echoes after each excitation. Furthermore, both b and c demonstrate that the 3D segmented EPI scheme improves the delineation of small arteries (such as peripheral vessels).

[0124] Figure 7 A schematic diagram comparing MIP images obtained using the existing 3D-GRE-based TOF imaging solution and the center-reordered 3D segmented EPI-based TOF imaging solution provided by the second embodiment of the present invention is provided.

[0125] a includes the upper, middle, and lower images in the left column, corresponding to the existing 3D-GRE-based TOF imaging solution with an echo acquisition time of TA = 4 min 25 s (4 minutes and 25 seconds);

[0126] b includes the upper, middle and lower images in the right column, corresponding to the 3D segmented EPI TOF imaging solution based on linear reordering provided by the second embodiment of the present invention, and the echo acquisition time TA = 2 min 12 s (2 minutes and 12 seconds).

[0127] from Figure 7 From a and b in the figure, we can see that:

[0128] Figures a and b show similar intracranial vascular appearances. However, the SNR in b is lower because the 3D segmented EPI sequence in b uses a higher bandwidth. Furthermore, the white arrows in b indicate that the 3D segmented EPI approach better depicts small arteries (such as peripheral vessels).

[0129] In summary, the TOF imaging scheme based on 3D segmented EPI using partial Fourier and the TOF imaging scheme based on 3D segmented EPI using center reordering provided by the embodiments of the present invention can reduce the echo acquisition time by about half compared to the traditional TOF imaging scheme based on 3D-GRE, and the image quality and contrast are comparable to those of the TOF imaging scheme based on 3D-GRE. At the same time, the TOF imaging scheme based on 3D segmented EPI using partial Fourier and the TOF imaging scheme based on 3D segmented EPI using center reordering provided by the embodiments of the present invention can better depict small arteries such as peripheral blood vessels.

[0130] Figure 8This is a schematic structural diagram of a TOF imaging device provided in a first embodiment of the present invention, which mainly includes:

[0131] Flow compensation module 81: In multi-shot EPI imaging, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for each echo of the excitation;

[0132] Echo acquisition module 82: Performs two scans for each excitation. These two scans use readout gradients of the same or opposite polarity to acquire echoes that have undergone flow compensation by flow compensation module 81. Each acquisition starts from the mth echo of the excitation and ends at the last echo, where m = N - "α*N" + 1, where N is the total number of echoes per excitation, "" is a round-up operator, and α is the partial Fourier parameter value in the phase encoding direction, where α < 1.

[0133] Echo averaging module 83: for each excitation, averages the echoes at the same position acquired by the echo acquisition module 82 twice to obtain the echo data corresponding to this excitation;

[0134] Imaging module 84 performs TOF imaging on the echo data corresponding to each excitation obtained by echo averaging module 83 .

[0135] In an optional embodiment, the echo acquisition module 82 uses α≥4 / 8.

[0136] In an optional embodiment, the values of N and α satisfy: N is an odd number, and "α*N" is an odd number. In an optional embodiment, the flow compensation module 81 performs flow compensation in the slice encoding direction, phase encoding direction, and frequency encoding direction for the echoes of each excitation, including: performing flow compensation in the slice encoding direction and phase encoding direction for the central echo of each excitation, the central echo being the central echo of multiple echoes to be collected after each excitation when a linear reordering method is used; and performing flow compensation in the frequency encoding direction for the odd-numbered echoes of each excitation.

[0137] In an optional embodiment, the flow compensation module 81 performs flow compensation in the slice encoding direction and the phase encoding direction for the central echo of each excitation, including:

[0138] Flow compensation in the layer selection coding direction and phase coding direction is performed according to the following formula:

[0139] M 1,par =M 0,par Δt par

[0140]

[0141] Among them, M 1,paris the first-order moment of the layer selection coding direction at the center echo, M 1,phase is the first-order moment of the phase encoding direction at the center echo; M 0,par is the zero-order moment of the layer-selected coding gradient in the layer-selected coding direction, M 0,phaseprephase is the zero-order moment of the pre-phase gradient in the phase encoding direction, M 0,par 、M 0,phaseprephase is different in different excitations; Δt par Δt is the time from the center of the layer coding gradient to the center of the echo in the layer coding direction, p Δt is the time from the center of the pre-phase gradient to the center of the echo in the phase encoding direction, pk M is the time from the encoding gradient of the kth echo in the phase encoding direction to the central echo; 0,pk is the zero-order moment of the phase encoding gradient of the k-th echo of a shot, k center It is the sequence number of the center echo to be collected when the linear reordering method is used.

[0142] Figure 9 This is a schematic structural diagram of a TOF imaging device provided in a second embodiment of the present invention, which mainly includes:

[0143] Flow compensation module 91: in multiple-shot EPI imaging, performs flow compensation in the slice encoding direction, phase encoding direction, and frequency encoding direction for each shot echo;

[0144] The echo acquisition module 92 determines a plurality of echoes to be acquired after each excitation, which have been flow compensated by the flow compensation module 91, when a linear reordering method is used. For each excitation, each echo is acquired starting from the center echo of the determined plurality of echoes in the positive or negative direction of k-space. The echo acquisition direction for the current excitation is opposite to the echo acquisition direction for the previous excitation.

[0145] Imaging module 93 performs TOF imaging on the echo data collected by the echo collection module 92 .

[0146] In an alternative embodiment, the echo acquisition module 92 acquiring the echoes of the current excitation in a direction opposite to that of the previous excitation includes: when the direction of acquiring the echoes of the previous excitation is: starting from the central echo and acquiring towards the positive k-space to the last echo equivalent to that acquired in the linearly reordered manner, the direction of acquiring the echoes of the current excitation is: starting from the central echo and acquiring towards the negative k-space to the first echo equivalent to that acquired in the linearly reordered manner; or when the direction of acquiring the echoes of the previous excitation is: starting from the central echo and acquiring towards the negative k-space to the first echo equivalent to that acquired in the linearly reordered manner, the direction of acquiring the echoes of the current excitation is: starting from the central echo and acquiring towards the positive k-space to the last echo equivalent to that acquired in the linearly reordered manner.

[0147] In an alternative embodiment, the echo acquisition module 92 acquiring each echo starting from the central echo of the determined multiple echoes towards the positive or negative k-space includes:

[0148] When the slice selection encoding gradient pulse and the phase encoding gradient pulse of the current excitation are transmitted, each echo is acquired starting from the central echo of the multiple echoes that have been flow compensated by the flow compensation module 91 towards the positive or negative k-space.

[0149] In an alternative embodiment, after the echo acquisition module 92 acquires each echo starting from the central echo of the determined multiple echoes towards the positive or negative k-space, it further includes: averaging the superposition of the central echoes of the m-th and (m + 1)-th acquisitions, where 1 ≤ m < M and M is the total number of excitations for the current EPI imaging.

[0150] In an alternative embodiment, the flow compensation module 91 performing flow compensation in the slice selection encoding direction, phase encoding direction, and frequency encoding direction for the echoes of each excitation includes: performing flow compensation in the slice selection encoding direction and phase encoding direction for the central echo of each excitation, where the central echo is the central echo of the multiple echoes to be acquired when using the linearly reordered manner after each excitation; performing flow compensation in the frequency encoding direction for the odd-numbered echoes of each excitation.

[0151] In an alternative embodiment, the flow compensation module 9's performing flow compensation in the slice selection encoding direction and phase encoding direction for the central echo of each excitation includes:

[0152] M 1,par = M 0,par Δt par

[0153] M 1,phase = M 0,phaseprephase Δt p

[0154] where M1,par is the first-order moment of the layer selection coding direction at the center echo, M 1,phase is the first-order moment of the phase encoding direction at the center echo; M 0,par is the zero-order moment of the layer-selected coding gradient in the layer-selected coding direction, M 0,phaseprephase is the zero-order moment of the pre-phase gradient in the phase encoding direction, M 0,par 、M 0,phaseprephase is different in different excitations; Δt par Δt is the time from the center of the layer coding gradient to the center of the echo in the layer coding direction, p is the time from the center of the pre-phase gradient to the center of the echo in the phase encoding direction.

[0155] The magnetic resonance imaging system provided in the embodiment of the present invention may include the TOF imaging device provided in the first embodiment or the second embodiment.

[0156] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A time-of-flight (TOF) imaging method, characterized in that: include: In multiple-shot echo planar EPI imaging, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for each shot echo. Two scans are performed for each excitation, and the two scans use readout gradients of the same or opposite polarity to acquire flow-compensated echoes. Each acquisition starts from the mth echo and ends at the last echo, where m = N - "α*N" + 1, where N is the total number of echoes acquired per excitation using the linear reordering method, "" is the ceiling operator, and α is the partial Fourier parameter value set in the phase encoding direction, and α<1; For each excitation, the echoes at the same position acquired during the two scans are averaged to obtain the echo data corresponding to this excitation; The TOF imaging is performed on the echo data corresponding to each excitation.

2. The method according to claim 1, characterized in that α≥4 / 8。 3. The method according to claim 1 or 2, characterized in that The values of N and α satisfy: N is an odd number, and "α*N" is an odd number.

4. The method according to claim 3, characterized in that The flow compensation in the slice encoding direction, phase encoding direction and frequency encoding direction for each excited echo includes: Performing flow compensation in the slice encoding direction and the phase encoding direction for the central echo of each excitation, wherein the central echo is the central echo of multiple echoes to be collected after each excitation when a linear reordering method is used; Flow compensation in the frequency encoding direction is performed on the odd-numbered echoes of each excitation.

5. The method according to claim 4, characterized in that The flow compensation in the slice selection encoding direction and the phase encoding direction for the central echo of each excitation includes: Flow compensation in the layer selection coding direction and phase coding direction is performed according to the following formula: M 1,par =M 0,par Δt par Among them, M 1,par is the first-order moment of the layer selection coding direction at the center echo, M 1,phase is the first-order moment of the phase encoding direction at the center echo; M 0,par is the zero-order moment of the layer-selected coding gradient in the layer-selected coding direction, M 0,phaseprephase is the zero-order moment of the pre-phase gradient in the phase encoding direction, M 0,par 、M 0,phaseprephase is different in different excitations; Δt par Δt is the time from the center of the layer coding gradient to the center of the echo in the layer coding direction, p Δt is the time from the center of the pre-phase gradient to the center of the echo in the phase encoding direction, pk M is the time from the encoding gradient of the kth echo in the phase encoding direction to the central echo; 0,pk is the zero-order moment of the phase encoding gradient of the k-th echo of a shot, k center It is the sequence number of the center echo to be collected when the linear reordering method is used.

6. A time-of-flight (TOF) imaging method, characterized in that: include: In multiple-shot echo planar EPI imaging, flow compensation is performed in the slice encoding direction, phase encoding direction, and frequency encoding direction for each shot echo. Determining a plurality of echoes to be acquired after each excitation when a linear reordering method is used, and for each excitation, acquiring each echo starting from a central echo of the plurality of echoes in a positive or negative direction of k-space, wherein the echo acquisition direction for the current excitation is opposite to the echo acquisition direction for the previous excitation; Perform TOF imaging on the collected echo data.

7. The method according to claim 6, characterized in that The echo collection direction for the current excitation is opposite to the echo collection direction for the previous excitation, including: When the echo acquisition direction for the previous excitation is: starting from the central echo to acquire in the positive direction of k-space to the last echo when acquired in a linear reordering manner, the echo acquisition direction for the current excitation is: starting from the central echo to acquire in the negative direction of k-space to the first echo when acquired in a linear reordering manner; or, When the echo acquisition direction for the previous excitation is: starting from the central echo to the negative direction of k-space to the first echo when acquired in a linear reordering manner, the echo acquisition direction for this excitation is: starting from the central echo to the positive direction of k-space to the last echo when acquired in a linear reordering manner.

8. The method according to claim 6, characterized in that The collecting each echo in a positive or negative direction of the k-space starting from the central echo of the plurality of echoes includes: When the selected slice encoding gradient pulse and the phase encoding gradient pulse for the current excitation are transmitted, starting from the central echo of the multiple echoes, each echo is collected in the positive or negative direction of the k-space.

9. The method according to claim 6, characterized in that Before performing TOF imaging on the collected echo data after collecting each echo starting from the central echo of the multiple echoes in the positive or negative direction of the k-space, it further includes: Averaging the superposition of the central echoes collected in the m-th and (m + 1)-th acquisitions, where 1 ≤ m < M, and M is the total number of excitations for the current EPI imaging.

10. The method according to any one of claims 6 to 9, characterized in that: The flow compensation for the echo of each excitation in the slice encoding direction, phase encoding direction, and frequency encoding direction includes: Performing flow compensation in the slice encoding direction and phase encoding direction for the central echo of each excitation, where the central echo is the central echo of the multiple echoes to be collected when using the linear reordering method after each excitation; Performing flow compensation in the frequency encoding direction for the odd-numbered echoes of each excitation.

11. The method according to claim 10, characterized in that The flow compensation for the central echo of each excitation in the slice encoding direction and phase encoding direction includes: M 1,par =M 0,par Δt par M 1,phase =M 0,phaseprephase Δt p Among them, M 1,par is the first-order moment of the layer selection coding direction at the center echo, M 1,phase is the first-order moment of the phase encoding direction at the center echo; M 0,par is the zero-order moment of the layer-selected coding gradient in the layer-selected coding direction, M 0,phaseprephase is the zero-order moment of the pre-phase gradient in the phase encoding direction, M 0,par 、M 0,phaseprephase is different in different excitations; Δt par Δt is the time from the center of the layer coding gradient to the center of the echo in the layer coding direction, p is the time from the center of the pre-phase gradient to the center of the echo in the phase encoding direction.

12. A time-of-flight (TOF) imaging device, characterized in that: Including: Flow compensation module (81): In the echo planar EPI imaging with multiple excitations, performing flow compensation in the slice encoding direction, phase encoding direction, and frequency encoding direction for the echo of each excitation; Echo acquisition module (82): Performing two scans for each excitation, where the two scans use readout gradient pairs with the same or opposite polarities to collect the compensated echoes, and each collection starts from the m-th echo of the excitation and ends at the last echo, where m = N - ⌈α * N⌉ + 1, N is the total number of echoes of each excitation, ⌈⌉ is the ceiling operator, and α is the partial Fourier parameter value in the phase encoding direction, and α < 1; Echo averaging module (83): For each excitation, averaging the echoes at the same position collected in the two scans respectively to obtain the echo data corresponding to the current excitation; Imaging module (84): Performing TOF imaging on the obtained echo data corresponding to each excitation.

13. A time-of-flight (TOF) imaging device, characterized in that: Including: Flow compensation module (91): In the echo planar EPI imaging with multiple excitations, performing flow compensation in the slice encoding direction, phase encoding direction, and frequency encoding direction for the echo of each excitation; Echo acquisition module (92): Determining the multiple echoes to be collected when using the linear reordering method after each excitation, starting from the central echo of the multiple echoes for each excitation, collecting each echo in the positive or negative direction of the k-space, and the echo collection direction for the current excitation is opposite to that of the previous excitation; Imaging module (93): Performing TOF imaging on the collected echo data.

14. A magnetic resonance imaging system, characterized in that: Including the time-of-flight (TOF) imaging device according to claim 12 or 13.

Images